A variety of geometric filters are available consisting of fixed metallic and dielectric elements and having transmission and reflection characteristics as a function of wavelength for applied electromagnetic fields. In one basic design, a metallic mesh filter is provided for use in far-infrared optics, where wavelengths greater than 10.0 microns and frequencies below 3.times.10.sup.13 Hz are involved. In such devices thin grids are provided having a thickness much less than the mesh width.
Metallic mesh filters may be formed as an inductive grid where the structure is formed in a metallic mesh. A complementary grid, or capacitive grid, may also be formed where metallic squares are supported in a symmetric pattern on a dielectric film. The characterization of such grid structures as inductive or capacitive is derived from the lumped circuit elements comprising an equivalent circuit for approximating the filter response.
Metallic grid filters generally obtain either high-pass or low-pass filtering with respect to the transmitted wave. Conventionally, a high-pass filter transmits frequencies with less attenuation near the diffraction edge of the mesh geometric structure.
Conversely, a low-pass filter provides greater transmission at frequencies corresponding to wavelengths less than the width of a metallic square. The basic theory of operation of such structures is well known, such as may be found in R. Ulrich, "Far-Infrared Properties of Metallic Mesh and Its Complementary Structure", 7 Infrared Physics 37 (1967).
Bandwidth filters, either band-pass or band-stop, have been made based on a cruciform geometry. A band-pass filter, an inductive element, generally includes a metallic surface defining cruciforms exposing a dielectric backing sheet. Again, a complementary pattern may be formed comprising metallic cruciform elements on a planar dielectric sheet. The complementary structure defines a band-stop filter, a capacitive structure, relative to the transmission of incident radiation. A basic discussion of bandwidth filters may be found in Tomaselli, et al., "Far-Infrared Band-Pass Filters from Cross-Shaped Grids", 20 Applied Optic 1361 (1981).
In addition to geometric structures which interact with electromagnetic waves, phase transition materials exist with variable properties, generally switchable between metal and dielectric properties, for interacting with electromagnetic waves. Thermochromic, or thermoptic, materials may be forced to undergo a phase transition from a material having dielectric, or wave transmitting, characteristics to a metallic conductor, or wave reflecting, characteristics. In some materials, the phase transition occurs when the material is heated or cooled and in other materials a phase transition occurs when selected materials are subjected to stress or to applied electromagnetic fields.
One particularly useful group of materials includes vanadium dioxide (VO.sub.2) and other related vanadium transition films, and titanium oxides (Ti.sub.4 O.sub.7). U.S. Pat. No. 4,283,113 teaches some other useful phase transition materials. Suitable phase transition materials may obtain a transmission of incident radiation of 95% in the dielectric state, reducing to a transmission of about 0.01% after being switched to a metallic reflecting element.
Other materials which exhibit transition properties between the dielectric and metallic states may be selected from the rare earth monochalcogenides and the organometallics. For example, the rare earth monochalcogenide samarium sulfide exhibits a phase transition when subjected to induced stresses. Organometallics, such as tetracyanoquinodimethane (TCNQ), exhibit phase transitions when subjected to an electromagnetic field.
There also exist semiconductor materials, particularly direct-gap semiconductors such as gallium arsenide (GaAs), which can be substantially transparent, or dielectric, for wavelengths corresponding to photon energies less than the band-gap energy. Wavelengths corresponding to photon energies above the band-gap energy (typically less than 1 micron) are strongly absorbed by the material. The band-gap energy can also be supplied by two photons of longer wavelength which arrive simultaneously and are absorbed, an event known as two photon absorption. Two photon absorption at these longer wavelengths can act to vary the optical properties of the material relating to absorption and refractive index where free carriers are generated by the photon absorption.
Also, other semiconductor materials such as indium antimonide (InSb), mercury cadmium telluride (HgCdTe), and the like, can obtain a plasma effect where free electrons and holes behave like mobile charge carriers. Below the so-called plasma frequency, the plasma appears metallic and totally reflective to incident radiation. Using such material, an optically bi-stable switch can be formed with operating speeds in the nanosecond range. A general discussion of exemplary materials may be found in Miller, "Dynamic Nonlinear optics in Semiconductors: Physics and Applications", Laser Focus (July, 1983).
Many useful structures may be formed from the above materials, generally in structures where the switching characteristics of the material between transmission and reflection may be directly useful. However, the useful frequency range may be determined by the material itself. Likewise, very high localized power must be provided to obtain responsive switching times over an area useful to form a shutter. For example, it is estimated that a VO.sub.2 film of 5,000 angstroms thickness could require 10-15 Kw/cm.sup.2 to obtain a 1 microsecond switching time.
Multilayer film structures using thin films with switchable characteristics have been proposed to provide optical structures with unique characteristics. However, it is difficult to predict the composite characteristics. Further, the fabrication yield of material meeting a given performance specification is low.
These and other difficulties in the prior art are overcome by the present invention, and an improved dynamic interactive structure using switchable elements in an array of fixed elements is provided.